The present invention relates to the general field of monitoring a turbojet. More particularly, it relates to a method and a system making it possible automatically to detect that an airplane turbojet in flight has ingested an object.
In flight, it is possible for foreign bodies or component parts of the turbojet to be ingested by the turbojet and to give rise to damage inside it. In particular, the impact of an object against a fan blade runs the risk of damaging it. In the event of damage due to ingestion of bodies that are foreign to the turbojet (e.g. birds), the term used is foreign object damage (FOD). In the event of damage due to ingesting component parts of the turbojet (e.g. rivets, bolts, etc.), the term used is domestic object damage (DOD).
Various known solutions exists for detecting the presence of FOD or DOD. One of those solutions is based on measuring the vibration of a rotor of the turbojet in order to detect the appearance of unbalance following damage thereto by ingesting an object. In the event of unbalance being detected, the turbojet is subjected to visual inspection on the ground (e.g. by endoscopy): the presence of traces of an impact, bird feathers, or perforations in the blades at the location of the impact then make it possible to diagnose the presence of FOD or DOD.
Nevertheless, such a solution presents numerous drawbacks. In particular, it is not capable of detecting the presence of FOD or DOD that does not lead to any unbalance of the turbojet rotors (or to an unbalance that is too small to be detected). Furthermore, visual inspection of the turbojet requires intervention on the ground that needs to be programmed in advance and that can be time consuming.
Another known solution consists in using measurement instruments on board the airplane (such as radar) for detecting the presence of FOD or DOD. That solution nevertheless presents the drawback of requiring additional instrumentation that is expensive, compared with the expected saving, and that makes the airplane heavier.
Document US 2007/0250245 discloses a method of monitoring a turbojet that makes it possible to detect almost instantaneously whether a foreign body has damaged the turbojet. That method provides for real time comparison between the vibration level of a turbojet rotor and a predefined threshold. In the event of that threshold being crossed for a predetermined duration, then a maintenance warning is issued.
Nevertheless, that method presents numerous disadvantages. In particular, it is limited to detecting those kinds of FOD or DOD that give rise, at least temporarily, to deformation of a fan blade (the method processes only reduced vibratory data). A simple impact caused by ingesting a body without deforming a fan blade (whether temporarily or permanently) is therefore not detected by that method. Furthermore, in order for a maintenance warning to be issued, it is necessary for the threshold to be exceeded over some minimum duration. There is therefore a risk that certain kinds of FOD or DOD will not be detected by that method. Finally, the method does not make provision for avoiding false alerts by eliminating electronic disturbances from the signal.
Document U.S. Pat. No. 6,907,368 discloses a method of detecting the ingestion of component parts (DOD) by applying Fourier transform analysis to a signal representative of turbojet noise in operation. Although effective in detecting the presence of DOD, that method presents the drawback of not being able to avoid false alerts caused by disturbances that are not the result of ingesting component parts.